The present invention relates generally to the field of plasma arc keyhole welding and, in particular, to a new and useful method and apparatus for welding cylindrical workpieces to prevent misformed welds and typical defects such as voids and porosity at the starting and stopping positions.
Plasma arc welding (PAW) is an arc welding process that produces coalescense of metal by heating with a constricted arc between an electrode and the workpiece transferred arc) or between the electrode and a constricting nozzle (nontransferred arc). The welding arc is generally obtained from the hot, ionized plasma gas issuing from the torch. This plasma gas is usually supplemented by an auxiliary source of shielding gas. The shielding gas may be a single inert gas or a mixture of inert gases. Filler material may or may not be added.
Plasma arc welding uses a nonconsumable electrode. The torch has a nozzle that creates a gas chamber surrounding the electrode. The arc heats the gas fed into the chamber to a temperature where it becomes ionized and conducts electricity. The ionized gas is defined as plasma.
One type of plasma arc weld which can be made is a keyhole weld. The keyhole plasma arc welding process is a high power density welding process since the arc has the capacity to penetrate through a thickness of material and make welds autogenously. The power density of the plasma arc welding process is controlled by the plasma gas flow rate and weld current, and the higher the values of these parameters, the higher the power density. However, at too high a power density and plasma gas flow rate, the process will cut the material into two pieces rather than welding it into one piece.
With keyhole welds, many times a single pass weld is all that is required to join two workpiece elements together. However, it is most difficult to initiate and terminate the welding process and achieve a good weld without creating internal defects or overpenetrating the joint. The keyhole is essentially an open hole melted through the workpiece which has molten metal sides and is held open by the force exerted by the plasma gas flow. This results in an inherently unstable mechanism during the start and stop of the weld process, since the molten metal sides of the keyhole are formed from solid metal.
When performing girth welds on cylindrical workpieces, or when other arcuate or curved workpiece are plasma arc welded, the workpieces are often rotated while the welding torch remains stationary. Starting and stopping the keyhole is the most difficult part of plasma arc keyhole welding. It is not uncommon, when plasma arc keyhole welds are made on a rotating workpiece, for voids in the weld to be created at the starting and stopping points of the weld. The voids are created by inaccuracies in the ramp-up timing between the workpiece travel speed, current level in the torch, and the plasma gas temperature. With thinner walled (less than 0.250" wall) workpieces this is less of a problem than is the case with larger, heavier workpieces. It is more difficult with thicker workpieces to synchronize these ramp-up characteristics to within a fraction of a second to avoid creating voids in a keyhole weld.
A known method of synchronizing the ramp-up characteristics and inserting material at the weld finish to avoid voids in the weld is taught by U.S. Pat. No. 5,225,654. The ramp-up parameters are continuously controlled and adjusted to prevent voids in the weld.